1. Field of the Invention
The present invention relates to a honeycomb structural body, made of cordierite, which can be used as a catalyst carrier in an exhaust gas purification apparatus for an internal combustion engine and, particularly, to a honeycomb structural body with a high cell density.
2. Description of the Related Art
Catalyst carriers for exhaust gas purification apparatuses have conventionally employed a honeycomb structural body 9 such as those shown in FIGS. 6 and 7, wherein partition walls 90 made of cordierite or the like are arranged in a honeycomb fashion to form multiple cells 99. An exhaust gas purification function is exhibited by carrying an exhaust gas purifying catalyst 8 on the surface of the partition walls 90 of the honeycomb structural body 9.
Incidentally, demands have been increasing in recent years for exhaust purification apparatuses with further improved purification functions and smaller sizes.
In order to satisfy the two demands of improved purification function and smaller size, it is effective to raise the cell density (increase the number of cells/in2) of the honeycomb structural body, i.e. to increase the number of cells per unit area. This enlarges the cell surface area and increases the contact area between the exhaust gas and the catalyst.
To exhibit an effect of increased cells/in2 of the honeycomb structural body it is necessary to fully maintain the exhaust gas channel so that no pressure loss occurs. It is therefore essential to avoid clogging of the cells when the catalyst is loaded. However, in high-cells/in2 honeycomb structural bodies of 600 cells/in2 or greater, cell clogging tends to occur, during catalyst loading, more readily than in conventional honeycomb structural bodies of low cells/in2 (400 cells/in2 or lower).
Loading of the catalyst is accomplished by coating the partition walls of the honeycomb structural body with a catalyst slurry, prepared using catalyst components, and drying it. The above-mentioned clogging tends to occur more easily with a higher solid concentration of the catalyst slurry (hereunder referred to as xe2x80x9cslurry concentrationxe2x80x9d). Consideration has therefore been given to reducing the slurry concentration to prevent clogging. However, when it is simply attempted to achieve a low concentration for the catalyst slurry, too little catalyst is loaded by a single catalyst slurry application. It therefore becomes necessary to significantly increase the number of applications of the catalyst slurry, and this results in increased production cost.
The present invention, which has been accomplished in light of these circumstances, is aimed at providing a honeycomb structural body with a high cell density of 600 cells/in2 or greater, and excellent loading properties of the catalyst.
The present invention relates to a honeycomb structural body comprising a plurality of cells formed by providing partition walls composed mainly of cordierite, which has the chemical composition SiO2: 45-55 wt %, Al2O3: 33-42 wt %, MgO: 12-18 wt %, in a honeycomb fashion, the honeycomb structural body being characterized in that the cell density is at least 600 cells/in2 and the pore volume of the partition walls is at least 30%.
The honeycomb structural body of the invention has a high cells/in2, with a cell density of at least 600 cells/in2, as stated above. If the cell density is less than 600 cells/in2 the purification performance will not be significantly improved when the catalyst is loaded to construct the exhaust gas purification apparatus. A higher cell density is more preferred, but limitations exist due to production technology, etc.
In order to achieve a higher cells/in2, it is essential to reduce the thickness of the partition walls. Specifically, the partition wall thickness is preferably no greater than 150 xcexcm. When it exceeds 150 xcexcm there is a risk of increased flow resistance (pressure loss) of the gas. The thickness is thus preferably no greater than 100 xcexcm.
The pore volume of the partition walls, that is, the proportion of the volume constituted by fine pores per unit volume of the partition walls, is 30% or greater. If the pore volume is less than 30% there will be no improvement in the loading property of the catalyst, and it will require multiple repetitions of the step of coating the catalyst slurry containing the catalyst components. On the other hand, if the pore volume is too high the strength of the partition walls will be low.
The pore volume is therefore preferably 35-80%, with an upper limit of 50% being preferred.
The function of the invention will now be explained.
As mentioned above, the honeycomb structural body of the invention has a high cell density of 600 cells/in2 or greater and a partition wall pore volume of at least 30%. Thus, when the catalyst is loaded onto the partition walls, it is possible to both prevent cell clogging and ensure the catalyst loading amount.
Specifically, by increasing the pore volume of the partition walls from the conventional average of 25% to at least 30%, the catalyst loading amount is greater than that according to the prior art when a catalyst slurry of the same concentration is applied. It is thus possible to lower the slurry concentration for loading the same amount of catalyst according to the prior art.
A lower slurry concentration can reduce occurrences of cell clogging.
Consequently, even when achieving a high cell density of 600 cells/in2 or greater, if the pore volume of the partition walls is 30% or greater as explained above it is possible to prevent occurrence of cell clogging while ensuring the same catalyst loading amount as with the prior art.
According to the invention, therefore, it is possible to provide a honeycomb structural body with a high cell density of 600 cells/in2 or greater and with an excellent catalyst loading property.
The thickness of the partition walls is preferably no greater than 80 xcexcm. This allows the pressure loss in the fluid passing through the partition walls to be drastically reduced.
The average roughness Rz of the surface of the partition walls is preferably 1-5 xcexcm. An average roughness Rz outside of the range of 1-5 xcexcm can present the risk of lower adhesion of the catalyst to the partition walls. The reason for this is believed to be that the particle size of alumina powder used as a catalyst component is largely distributed in the range of 1-5 xcexcm.
The mean size of the fine pores formed inside the partition walls is preferably 1-10 xcexcm. If the mean size of the fine pores is less than 1 xcexcm there will be a problem in that the alumina powder will not enter into the fine pores and adhesion will thereby be reduced, while if it exceeds 10 xcexcm there will be a problem of reduced strength of the partition walls.
The honeycomb structural body may be a catalyst carrier wherein a catalyst is loaded onto the surface of the partition walls. The catalyst may be a ternary catalyst for purification of NOx, CO, HC, etc. Specific components include alumina powder, and mixtures of platinum, palladium, etc. In this case, the honeycomb structural body may be applied as a catalyst carrier in a catalytic converter for purification of automobile exhaust gas, for example.
For loading of the catalyst, the catalyst slurry concentration conditions will differ depending on the catalyst components that are loaded, but as an example, it is preferably carried out by coating the partition walls with a catalyst slurry having a solid concentration of no greater than 45%. If the solid concentration (slurry concentration) is greater than 45% a problem is presented of easier clogging in cells of 600 cells/in2 or greater. On the other hand, if the slurry concentration is too low the catalyst loading amount is reduced, and therefore the lower limit for the slurry concentration is preferably 40%.